How to get a high resolution image of the material preview?
You can adjust the DPI, zoom in, and full screen the Materials panel to make it bigger, then take a screenshot.

Make the Properties editor full screen by hovering over it and pressing Ctrl↑

Increase the DPI in User preferences &gt; System

Zoom in on the panel by holding CtrlMMB and dragging the cursor


Result: (click for full size)


The obvious limitation being that you can't go larger than the maximum zoom, or larger than your monitor.
Get the blend:
Alternatively, you can download the blendfile from the GIT repo with your browser via Phabricator:

Blender Internal preview file

Cycles preview file


To download a file, go to the link above and click Download raw file on the right:

Then click Download:

The different preview scenes are on the following layers:
Layer 1: Flat XY plane
Layer 2: Sphere
Layer 3: Cube
Layer 4: Suzanne (Monkey)
Layer 5: World Sphere
Layer 11: (the first layer on the second row) Hair